Amateur Sweating DWV fittings

I've had pretty good luck (as an amateur) sweating 1/2", 3/4" and occasionally even 1" copper for various DIY projects around my house. Recently, I attempted to sweat an 1 1/2" trap adapter to a double ell (?) DWV fitting, both (I believe) made of cast brass. I cleaned all surfaces very well and cleaned/fluxed with Oatey product that contains tin. I use mapp for heat. The joint was horizontal, so I applied heat to bottom of joint, and touched solder to top of joint. It seemed to take forever for the solder to melt/run, by which time the entire fitting was black, and the resulting joint looked awful (no way I would trust integrity of joint inside wall of our new bathroom). At first it seemed that I was not getting the assembly hot enough, but in the end, I am guessing that I eventually got it way too hot. What went wrong?

My guess would be: You were relying on the heat to travel at the same rate around the entire joint without moving the torch tip to the opposite side of the joint. If space allows, The heat should be distributed evenly around the entire fitting/joint before solder is applied.
1 1/2" takes a little longer to heat up

That is precisely what I did: I usually keep the torch stationary on one side of the joint and apply the solder from the opposite side, and it has always worked for 1/2" and 3/4" fittings, obviously these fittings have much less mass, and even heating has not been an issue. Thanks for the input.

If you were applying the solder properly, you would never have reached the point where the joint was "overheated" because the solder would have flowed long before you reached that point. There must be something wrong with your technique, since DWV is usually the easiest joint to solder.

Yes, no doubt it was technique. As krow stated above, moving the source of heat around the fitting to ensure even heating makes sense, but I never found the need to do that with the small copper fittings I have used in the past. Given the relative mass of the 1 1/2" brass/bronze double ell and trap adapter (as compared to, for example, a small 1/2" copper elbow), it makes sense that uneven heating was the cause of my problem.

Large pipe and fittings are not only a big heat sink but also a big radiator...to get things evenly heated up, you need to move the torch around, once it's hot, keep the solder out of the flame and apply.

i found that with 1 1/2 and 2 inch copper, that mapp alone did not get enough heat, i needed mapp in a higher temperature "turbo torch". Also, sometimes , I needed a second hand to hold a second torch to get the joint hot enough.